In a large number of applications the present trend of development is to replace electronic or opto-mechanical components by electro-optical or pure optical components based on non-linear optical ("nlo") effects, because in principle these operate faster, more reliably and more economically than the former. Many of these applications, e.g. optical storage systems with high data density, would require compact rugged laser light sources emitting in the range of&lt;500 nm. Since short-wave laser diodes are not available and have not hitherto been realizable, attempts have been made for a long time to convert long wave-length light from conventional laser diodes efficiently into short wave-length light by non-linear optical materials, particularly by frequency doubling ("Second Harmonic Generation: SHG"), summation of frequencies, etc. Techniques which permit the integration of the laser diode with the nlo active material in an integrated optical device would be particularly attractive.
Efficient frequency conversion can be obtained only if the "phase-matching" condition for the irradiated long wave-length light and the generated short wave-length light can be fulfilled in the nlo-active medium. This can be achieved with suitable non-linear crystals such as KDP, ADP, LiNbO.sub.3, etc.
Since the efficiency of conversion depends on the square of the power density of the fundamental wave and since the light intensity of available laser diodes is relatively small, impracticably large and expensive nlo crystals are required. The conditions are much more favorable if the conversion can be made in waveguide configurations instead of in the aforementioned crystals. In wave guiding configurations, light is compressed to a small cross-section, thus increasing the power density by several orders of magnitude. In addition, the waveguide geometry makes it easier to fulfill the phase-matching condition. The following are three possible phase matching techniques:
First if the use of the dispersion property of various modes in the waveguide. If the waveguide is correctly dimensioned, the phase-matching condition is fulfilled for the fundamental wave when guided in low-order modes and for the second harmonic guided in a higher mode. However, the conversion efficiency depends on the square of the overlapping integral between the two modes and is small for modes of different order.
Second is the use of the birefringence of anisotropic waveguides. This method corresponds to the situation in birefringent crystals and is applicable only to materials with suitable birefringence. In this case, conversion between modes of the same order (e.g. TE.fwdarw.TM) is possible, so that large values of the overlapping integral can be obtained.
Third is the use of periodic structures. With this method, known as quasi phase-matching, the non-linearity of the waveguide material is periodically modulated. This method has become important recently, since it is relatively easy to carry out in poled structures (e.g. poled nlo polymers). In periodic structures conversion also occurs between modes of the same order (e.g. TE.sub.o.sup.(.omega.) .fwdarw.TE.sub.o.sup.(2.omega.)) and is therefore highly efficient.
In the present state of the art, .chi..sup.(2) -active materials potentially suitable for the aforementioned waveguides are the following classes:
1. Monocrystalline inorganic layers.
One typical example is LiNbO.sub.3, which has been particularly intensively studied, inter alia because of its high non-linear coefficients d.sub.31 =-5.95 ppm/V and d.sub.33 .tbd.-37 pm/V, and is frequently used as a reference material for judging nlo substances. One disadvantage, however, is that LiNbO.sub.3 -layers are difficult to prepare and are very expensive.
2. .chi..sup.(2) -active Langmuir-Blodgett multilayers. These have the disadvantage that they are unstable, particularly with respect to temperature.
3. Poled .chi..sup.(2) -active polymers. Their disadvantage is the instability of their nlo-coefficients with time.